Heat-resisting wall structures



Jan. 5, 1960 L. HAWORTH ETAL 2,919,549

HEAT-RESISTING WALL STRUCTURES Filed Jan. 27. 1955 6 Sheets-Sheet 1 Jan. 5, 1960 L. HAWORTH L HEAT-RESISTING WALL STRUCTURES Filed Jan. 27. 1955 6 Sheets-Sheet 2 m El WAU 1.. HA W R 7 Jan, 5, 1960 L. HAWORTH ETAL HEAT-RESISTING WALL STRUCTURES 6 Sheets-Sheet 3 Fi -led Jan. 27. 1955 Jan. 5, 1960 L. HAWORTH EF-AL 2,919,549

HEAT-RESISTING WALL STRUCTURES Filed Jan. 27. 1955 70b 70 65 70a 70b 70 V07 11 65 n. W 711w. lmm

L.HA R T R JTfH/Rt 6 Sheets-Sheet 4 5, 1960 L. HAWORTH L 2,919,549

HEAT-RESISTING WALL STRUCTURES Filed Jan. 27. 1955 6 Sheets-Sheet 5 30a J7 J6 :59 10b IN EAI 7 0R 1.. HAWORTH R JTSHIRE Stte HEAT-RESISTING WALL STRUCTURES Application January 27, 1955, Serial No. 484,492

Claims priority, application Great Britain February 26, 1954 15 Claims. (Cl. 6039.65)

This invention relates to heat-resisting wall structures employed to define a space wherein fuel is burnt, or to define a duct wherein hot gas may flow, for example in a gas-turbine engine.

The invention has for an object to provide an improved heat-resisting wall construction whereby the possibility of failure of the wall, for example due to overheating or due to thermal shock, is reduced.

According to the present invention a heat-resisting wall structure defining a space wherein fuel is burnt is formed at least in part by a multiplicity of pieces arranged mosaicwise with their edges unjoined but close 'tO one another, said pieces being supported individually in juxtaposition from support structure externally of said space.

According to one preferred feature of the invention the pieces are supported from the support structure through means aflording paths of restricted heat conductivity.

In one preferred arrangement according to the invention the support structure comprises a substantially continuous wall, and the pieces provide a protective wall spaced'away from the continuous wall.

In another preferred arrangement according to the invention the support structure is of skeleton form, for example comprising a plurality of rings arranged in axially spaced relation on rod elements, the pieces of the mosaic wall being mounted individually on the skeleton support structure.

Preferably the heat-resisting wall structure is accommodated in a casing providing a passage for the flow of cooling air externally of the space defined by the heatresisting wall structure. Alternatively or additionally in constructions having asupport structureprovided by a substantially continuous Wall, provision may be made for creating a flow of cooling air between the continuous wall and the wall constituted by the pieces.

Preferably the pieces are of regular shape such as triangular, rectangular, square or hexagonal, so that when they are arranged mosaicwise they form an area of wall without large gaps between the pieces.

The pieces may be made either separately from the support structure and then be mounted individually thereon, or they may be made integral withthe support structure.

The pieces may be connected with the support struc ture each by one or more stalks or stems which have individually a small cross-sectional area at right angles to the direction of heat conduction as compared with the surface area of the piece. Preferably a single stalk or stem is employed for each piece. Alternately each piece may be connected with the support structure through a narrow Web; for instance a heat-resisting structural wall structure .may be formed from lengths or turns of H-section strip, the lengths or turns of the strip being joined by welding together the longitudinal edges of adjacentflanges to form a continuous external ,supportwall,andtheother flanges being slotted to divide ice the flange into a number of pieces, and the web of the H-section which connects the flanges also being slotted to divide the web into a number of sections, each section joining a single mosaic piece to the continuous structural wall.

if desired, provision may be made for passing a cool fluid from outside the wall structure to within the heated space, the cool fluid flowing into the heated space through the small gaps left between adjacent mosaic pieces. For instance, when the heat-resisting wall structure forms the wall of say a flame tube of gas-turbine engine combustion equipment, the wall portion may have apertures therein to admit secondary or tertiary air into the flame tube. Alternatively or additionally air may be metered to the combustion space through the gaps between the mosaic pieces. In constructions in which a continuous outer wall structure is used the relative restrictions of the apertures in the wall structure and the gaps between the mosaic pieces will preferably be selected so that the pressure between the inner and outer wall structures lies between the pressure outside the wall portion and the pressure within the flame tube.

If desired, stalks or stems by which the mosaic pieces are connected to the outer support structure may be formed with ducts extending through them to permit the passage of a cool fluid from outside the wall portion to within the heated space and also to reduce the effective cross-sectional area available for conduction.

Some applications of the invention to the walls of flame tubes of gas-turbine engine combustion equipment will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 illustrates in perspective a section of a flame tube wall,

Figures la, 1b, 1c, 1d show alternative methods to that illustrated in Figure 1 for securing parts of the wall structure together,

Figures 1e, 1 and lg show alternative forms for parts of the protective screen,

Figure 2 shows an alternative form of wall structure,

Figure 2a illustrates the method by which it may be made,

Figure 3 illustrates yet another form for a wall structure,

Figure 3a illustrates a method by which it may be formed,

Figure 4 illustrates yet another form of a wall structure,

Figure 5 illustrates partly in section a portion of a flame tube with a heat-resisting wall portion made in yet another form according to the invention,

Figure 5a illustrates a detail of Figure 5,

Figure 6 illustrates diagrammatically a flame tube having a support structure of skeleton form with sheetmetal Wall pieces supported thereon,

Figure 6a is a section on the line a-a of Figure 6,

Figure 6b is an enlarged view of a portion of the section shown in Figure 6a,

Figures 7 and 7a show a variant of the construction shown. in Figures 6, 6a and 6b in which the wall pieces are provided with interlocking tongues and grooves,

Figure 8 is a view corresponding to Figure 6 of another construction of flame tube,

Figure 8a is a part of Figure 8 to a larger scale,

Figure 8b is a section on bb of Figure 8,

Figure 8c is a view on the outside of the fiame tube of Figure 8, and

Figure 9 is a part of another form of flame tube.

Referring to Figure l, the heat-resisting wall structure forms part of a flame tube of a gas-turbine engine combustion equipment which comprises an outer wall 10 which in use will provide one boundary for the air passage between the air casing 10a of the combustion equipment and the flame tube. The outer wall is substantially continuous but may have formed in it apertures such as the apertures 11 which may serve for the passage of ignition devices, fuel injectors of flame tube interconnectors. The apertures 11 may be of any convenient shape such, for example, as shown in Figures 1e, 1 and lg according to the purpose which they are to serve.

The heat-resisting wall structure also comprises an inner wall structure affording a protective screen for the outer wall which supports it. The inner wall comprises a mosaic of pieces 12 which are spaced away from the outer wall and which in use afford a boundary for the combustion space of the flame tube. Each piece 12 is supported from the outer wall 10 by an individual stalk or stem 13 which may take a number of convenient forms. For instance, in Figures 1, 1a, and lb, the stalk or stem 13 is formed integral with its associated mosaic piece 12 and projects through an aperture in the outer wall 10, which aperture has a peripheral neck 14 produced in the aperture-forming operation and the stalk or stem 13 is welded to the outer wall 10 on the side thereof remote from the mosaic pieces 12. In Figure 1d the stalk or stem 13 is similar to the stalk or stem shown in Figures 1, 1a and lb, but in this case is copper brazed to the outer wall 10 on the side thereof adjacent the combustion space of the flame tube. In Figure 1c the stalk or stem 13 has a rivet connection 15 with the outer wall 10. The stalk or stem 13 may be solid as shown in Figure la or may have as shown in Figure 1 a drilling 16 through it for the passage of cooling air, or may have a venturishape passage 17 through it as shown in Figures 1b, la and 1d. By supporting the mosaic pieces 12 of the inner wall in this way they are held in spaced relation to the outer wall 10.

The mosaic pieces are individually substantially smaller in area than the outer wall.10, so that a multiplicity of mosaic pieces 12 is required to form the protective screen for the outer wall portion 10. The pieces 12 are conveniently made of regular shape such as square as shown in Figures 1 and 1e, or hexagonal as shown in Figure 1f, or triangular as shown in Figure lg, so that their edges may be readily brought close together and so that the spacing of the edges may be readily maintained equal.

Where an aperture 11-is provided in the outer wall, a corresponding aperture is formed in the inner wall structure, for instance by omitting one or more of the mosaic pieces 12. Preferably the aperture in the outer wall 10 is larger than that left by omission of the mosaic pieces 12.

The stalks or stems 13 have a small cross-sectional area as compared with the area of the pieces 12 and so afford a relatively low heat conductivity path between the pieces 12 of the inner wall structure and the outer wall 10.

Where the mosaic pieces 12 for forming the inner wall structure are made separately from the outer wall 16, the mosaic pieces 12 may be formed from a heatresistant material, for instance from the metal marketed under the trademark Nimonic or from a ceramic, and the outer wall may be formed from a material such as stainless steel or mild steel selected according to the temperature it is likely to experience in operation. Preferably the material is selected so that the conductivity of the outer wall is high.

Referring now to Figures 2 and 2a, there is illustrated a method of producing a wall structure whereof the mosaic pieces forming the inner wall structure are integral with the outer supporting wall. A thick plate is taken and a series of drillings made in the thickness of the plate parallel to the surfaces thereof, there being for example one set of drillings 20 in one direction and produced as by a drill 20a, and a second set of drillings 21 at right angles to the first set of drillings 20 and produced as by a drill 21a.

There are thus formed a series of stalks or stems 22 interconnecting the undrilled surface portions of the plate.

One of the surface portions is now machine to produce a series of slots 23 parallel to the axes of the drillings 20 and contained in planes through the respective axes of the drillings 20 and at right angles to the surfaces of the plate, and a second series of slots 24 parallel to the axes of the drillings 21 and contained in planes through these axes and at right angles to the surfaces of the plate. The machined surface is thus divided up into a series of square pieces 25 whereof the edges are unjoined but close to one another and which are joined to a substantially continuous wall 26 afforded by the unmachined surface portion by the stalks or stems 22.

In the arrangement illustrated in Figures 2 and 2a, the spacing of the drillings 21 is the same as the spacing of the drillings 20, but clearly if the spacings of the drillings 20 and 21 difler, then the pieces 25 will be rectangular and if the inclination of the drillings 21 with respect to the drillings 20 is other than a right angle, then the pieces 25 will be of parallelogram form. If, say, triangular pieces are required, then a further set of drillings will be necessary.

Referring now to Figures 3 and 3a, there is illustrated another arrangement in which the mosaic pieces are integral with the inner wall.

In this arrangement the heat-resisting wall structure is formed from extruded metal strip of H-section and lengths of the strip are brought into abutment over the longitudinal edges of one set of flanges 3t) and are welded together over these flanges. The other set of flanges 31 are left unconnected and a series of saw-cuts 32 are made across these flanges at right angles to the longitudinal axes of the lengths of H-section strip so as to divide the flanges 31 into a number of pieces 33 which are unconnected at their edges.

A series of drillings are made by a drill 34 at right angles to the length of the strips so as to divide the webs 35 joining the flanges 30, 31 into a number of individual web pieces each corresponding to a single mosaic piece 33. The holes 36 made by the drill 34 have their axes parallel to and in the same planes as the series of sawcuts 32.

It will be seen that in this arrangement also the structure has a substantially continuous support wall formed by the flanges 30 and an inner wall formed by the pieces 33 which protects the structural wall.

Referring now to Figure 4, there is illustrated another arrangement and in this arrangement there is first produced a sheet 40 having thereon a series of parallel fins which are slotted at right angles to their length, as indicated at 41, to produce a series of short web elements 42. This sheet has attached to it a second sheet 43 as by welding and a series of intersecting slots 44 are then machined in the sheet 40 to produce mosaic pieces 45, each mosaic piece 45 being associated with a web element 42. One set of the slots 44 is machined parallel to but between the web elements 42, and a second series of slots is machined at right angles to the web elements and adjacent the slots 41.

Referring now to Figures 5 and 5a, there is illustrated the inlet end and intermediate portion of a flame tube of gas-turbine engine combustion equipment, whereof part of the wall of the flame tube is a mosaic wall structure in accordance with the invention.

In the arrangement illustrated the portion of the wall of mosaic form is produced in a manner similar to that illustrated with reference to Figures 3 and 3a except that instead of employing a number of lengths of H-section strip a single strip is used and is wound spirally. The same reference numerals are used in Figures 5 and 5a as are employed in Figures 3 and 3a.

Referring now to Figures 6, 6a, and 6b there is illustrated the inlet end, intermediate portion and outlet end of a flame tube of gas-turbine engine combustion equiphgent. The inlet endisshown at 60 and the outlet end at 61. The intermediate portion comprises a skeleton support structure provided by a number of rods 62 which are welded or otherwise secured at their ends to ring members 63 and 64 supported respectively by the inlet end 60 and the outlet end 61. The rods 62 extend parallel to the axis of the flame tube and are disposed on a circle having its centre on the said axis. The spacing of the rods intermediate the end portions 60 and 61 is maintained by rings 65 which have apertured lugs 65a through which the rods 62 pass. Preferably the rings 65 are ,welded or otherwise secured to the rods 62. The rods 62 and rings 65 thus form a skeleton support structure providing a rigid connection between the inlet and outlet portions 66 and 61.

' Sheetmetal wall pieces 66 are supported on this skeleton structure, being of substantially rectangular form and having tabs upstanding from their outer surfaces and hooked or bent over the rods 62. At one end of each wall piece there is a single central tab, whilst at the other end there are two such tabs spaced apart. In the assembly the first tab engages a rod 62 between the two tabs of the adjacent wall piece. A suitable tool is used for turning over the end portions to engage the rods 62.

In flame tube constructions of gas-turbine engines it is often necessary to provide duct connections in the wall of the flame tube. Thus as shown in Figures 6 and 6a an interconnector pipe 67 is provided, this pipe being supported on a wall piece 68 the area of which is approximately four times the area of a standard wall piece. In the region of the interconnector one of the rods 62 'is discontinued, whereby the wall piece 68 spans an extended area, being connected to the support structure in a manner similar to the standard wall pieces.

The construction described provides a wall structure in which the individual pieces have relative freedom for thermal expansion, whilst the support structure lies externally of the fuel burning space and is therefore relatively cool. In addition such structure is in the installation of the flame tube in a gas-turbine engine subjected to a cooling air flow which passes between the flame tube wall and an outer air casing containing the flame tube.

In the variant shown in Figures 7 and 7a the wall pieces are shown to be of a form which may conveniently be produced by casting methods. Thus each piece 70 is formed with a lug portion 7% which is drilled to receive a rod 62. In addition the edge of the wall piece 70 adjacent the lug 76a is grooved at 70b to receive a tongue formation on the edge of the adjacent wall piece. In the case of the arrangement shown in Figure 7 it will be necessary to thread the individual wall pieces on to rods 62 before welding rings such as shown in Figure 6 at 65 on to the rods. The assembly may thus for example involve securing the rods 62 to one end portion, e.g. the inlet 69, threading on say two rows of wall pieces, assembling a ring such as 65 on to the rods 62 and securing it in place, and thereafter adding further sets of wall pieces and rings 65 until the assembly is completed by securing the end portion 61 to the rod 62. As diagrammatically illustrated in Figure 7 the wall pieces may overlap in the axial sense by providing an undercut on one wall piece engaged by a protruding tongue on the co-operating wall piece of the adjacent row. In this manner the leakage flow of gas through the wall may be reduced.

Referring now to Figures 8, 8a, 8b, 80, there is illustrated another view of flame tube for use in gas-turbine engine combustion equpment. In this figure the outer aircasing is also shown at Itib. The construction is similar to that shown in Figures 5 and 5a, in that it is made from H-section strip but in this case instead of the strip being wound spirally there are a number of lengths'of strip arranged side by side to produce a cylinder and the strips are joined together over the longitudinal edges of theirouter flanges 30 as by welding.

The inner flanges of the lengths of stripare left unconnected along their edges and are divided by circumferentially extending slots 32a into a plurality of mosaic pieces 33. The slots are extended radially outwards across the webs 35 which join the mosaic pieces 33 and the outer flanges 30.

Adjacent their upstream ends, the lengths of strip have their inner flanges and the webs removed to leave portions 30a which are curved inwardly to form part of the entry section of the flame tube.

Air may be admitted to the spaces between the webs 35 by means of holes 37 in the outer flanges 30.

The spaces 35 may be divided up into a number of axially separated sections by forming circumferential slots in the outer flanges 30 to extend radially inwards through the webs 35 and by welding annuli 38 in position in the slots so as to form between the axially separated sections.

Holes may be provided through the heat-resisting wall structure for the admission of air, for the interconnection of flame tubes by balance pipes, or for other purposes. These holes are conveniently fitted with liners, for instance as indicated at 39, and the sleeves may be of greater diameter where they are secured to the outer flanges 30 of the lengths of H-section strip than adjacent the mosaic pieces 33.

Referring now to Figure 9, there is illustrated another construction and in this construction H-section rings are employed and these may have attached to them, as by welding, in abutment with their outer flanges 71 tubular parts 72. The inner flanges of the H-section rings are divided up into mosaic pieces as indicated at 73, and the webs joining the outer flanges 71 with the mosaic pieces 73 are slotted as indicated at 74 so that each mosaic piece 73 is supported by an individual web section. In the construction shown in Figure 9 the central tubular part 72 may of course be replaced by other H-section rings. Holes 75 are provided in the outer flanges 71 to allow air to flow to between the flanges 71 and mosaic pieces 73.

In arrangements according to the invention such as described above size of the mosaic parts will be determined in any particular case and ordinarily the size will be a compromise between conflicting design considerations. In view of the square cube law, the edges of the mosaic pieces will tend to droop under their own weight when very hot if made larger than a certain size, and from this point of view the mosaic pieces should be kept as small as possible. However, in order to reduce cost of manufacture and complication the mosaic pieces should be kept as large as possible.

A square is thought to be a suitable size for the mosaic pieces, and 0.15" is thought to be a suitable spacing for the mosaic pieces from the outer wall.

It is thought that for instance flame tubes manufactured in accordance with this invention will have an increased life, which is desirable since in modern gasturbine engines a flame tube has a shorter life than other parts such as the highly stressed moving blades of a turbine.

In the foregoing examples in which the heat-resisting wall structure forms part of a flame tube, the supply of secondary and tertiary air into the combustion space is conveniently effected through the heatresistant wall from the space immediately within the air casing structure. To this end apertures are formed in the outer support wall structure of such size in relation to the gaps between the mosaic pieces, that the pressure between the outer wall and protective screen is intermedate thg pressures outside the outer wall and within the flame tu e.

The mosaic screen connected to the outer wall by stalks, stems or narrow webs has the eifect of prevent-- vection and substantially reduces the rate at which heat can be conveyed to the outer wall by conduction. Thus, in the application of the invention to flame tubes, the outer wall of the flame tube may be made thinner and of a material appropriate to the much lower temperature that it will experience in operation.

Another advantage of the arrangement of the invention is that if a piece of the protective screen breaks away and is carried through the turbine, the possibility of damage to the turbine is low because of the small size of the mosaic pieces.

We claim:

1. Gas-turbine engine combustion equipment comprising an outer air casing structure which contains a flame tube made up from a set of lengths of H-section strip which strip comprises a pair of end flanges interconnected by a web formation, the one end flanges affording a first wall of the flame tube which defines a combustion space and the other end flanges being joined together longitudinally to provide a continuous support wall on the side of the first wall remote from the combustion space, pressure cooling air being fed to the spaces between the walls, said one end flanges being slotted transversely to provide a multiplicity of relatively small pieces arranged mosaicwise and having surfaces thereof facing the combustion space contained in a substantially continuous geometrical surface, the edges of adjacent pieces being unjoined but close to one another, and at least those edges of adjacent pieces which extend in directions transversely to the general direction of gas flow through the combustion equipment affording between them restricted air leakage flow passages into the combustion space, whereby the air leaking through said passages flows substantially over the whole of the surface of said second wall facing the combustion space.

2. Gas-turbine engine combustion equipment as claimed in claim 1, having the transverse slots in said one end flanges extending into the web formations of the lengths of H-section strip, whereby the webs are divided to afford individual stalks connecting the small pieces to said continuous support wall.

3. Gas-turbine engine combustion equipment as claimed in claim 1, wherein the lengths of H-section strip together form a tube and have their longitudinal edges parallel to the tube axis, the lengths of H-section strip having said other end flanges joined together edge to edge to form said continuous support wall and having said one end flanges and their web formations divided by slots to form said small pieces and individual stalks connecting the small pieces to the continuous support wall.

4. Gas-turbine engine combustion equipment as claimed in claim 3, wherein the lengths of H-section strip have axially-spaced circumferential slots extending radially through said other end flanges and the web formations and wherein annuli are accommodated in these slots and are secured by a fuse-d metal joint to said other end flanges thereby to divide the spaces between the web formations into axially-separated sections.

5. Gas-turbine engine combustion equipment as claimed in claim 1, comprising holes in said other end flanges to place the spaces between the webs in communication with the exterior of said other end flanges, for the flow of cooling gas from the space immediately within the outer air casing structure to the combustion space.

6. Gas-turbine engine combustion equipment comprising an outer air casing structure which contains a flame tube made up from turns of H-section strip which strip comprises a pair of end flanges interconnected by a web formation, the one end flanges affording a first wall of the flame tube which defines a combustion space and the other end flanges aflording a continuous support wall on the side of the first wall remote from the combustion space, pressure cooling air being fed to the spaces between the walls, said one end flanges being slotted transversely to provide a multiplicity of relatively small pieces arranged mosaicwise and having surfaces thereof facing the combustion space contained in a substantially continuous geometrical surface, the edges of adjacent pieces being unjoined but close to one another, and at least those edges of adjacent pieces which extend in directions transversely to the general direction of gas flow through the combustion equipment affording between them restricted air leakage flow passages into the'combustion space, whereby the air leaking through said passages flows substantially over the whole of the surface of said second wall facing the combustion space.

7. Gas-turbine engine combustion equipment as claimed in claim 6, having the transverse slots in said one end flanges extending into the web formations of the lengths of H-section strip whereby the webs are divided to afford individual stalks connecting the small pieces to said continuous support wall.

8. Gas-turbine engine combustion equipment as claimed in claim 6, wherein the H-section strip is wound spirally into a tube.

9. Gas-turbine engine combustion equipment as claimed in claim 6, comprising holes in said other end flanges to place the spaces between the webs in communication with the exterior of said other end flanges, for the flow of cooling gas from the space immediately within the outer air casing structure to the combustion space.

10. Gas-turbine engine combustion equipment comprising a heat-resisting wall structure defining a combustion space which wall structure comprises at least one ring of material of H-section, the central web of the section extending radially and the two flanges of the section extending axially, one of said flanges of the ring or each ring being welded to adjacent annular portions of the wall structure so as effectively to form part of the wall structure, the web extending towards the combustion space from said one flange, and the other flange being slotted transversely to provide a multiplicity of pieces which are supported by the web, and air inlet holes being provided in the region of the wall structure adjacent said web for the passage of pressure cooling air from the exterior of the wall structure into the interior thereof.

11. Gas-turbine engine combustion equipment as claimed in claim 10 in which a plurality of rings of H-section material are provided adjacent one another with their corresponding flanges close together, the flanges remote from the combustion space being welded together.

12. Gas-turbine engine combustion equipment as claimed in claim 10, wherein at least two rings of H-seotion material are provided at axially spaced position of the structure, there being an annular wall member extending therebetween, said one flange of each ring being welded to the adjacent annular wall member or members.

13. Gas-turbine engine combustion equipment as claimed in claim 10, in which each web is additionally slotted down to the region of the flange remote from the combustion space.

14. Gas-turbine engine combustion equipment as claimed in claim 10, in which said air inlet holes are formed in the flange of the H-section ring which is remote from the combustion space.

15. Gas-turbine engine combustion equipment comprising an outer air casing structure which contains a flame tube made up of H-section strip which strip comprises a pair of end flanges interconnected by a web formation, the one end flange affording part at least of a first wall of the flame tube which defines a combustion space and the other end flange forming part at least of a continuous support wall on the side of the first wall remote from the combustion space, pressure cooling air being fed to the spaces between the walls, said one end flange being slotted transversely to provide a multiplicity of relatively small pieces arranged mosaicwise and having surfaces thereof facing the combustion space contained in a substantially continuous geometrical surface, the edges of adjacent pieces being unjoined but close to one another, and at least those edges of said pieces and adjacent parts of said first wall which extend in directions transversely to the general direction of gas flow through the combustion equipment affording between them restricted air leakage flow' passages into the combustion space, whereby the air leaking through said passages flows substantially over the whole of the surface of said second wall facing the combustion space.

10 Banker-t Nov. 24, 1936 Mahnken Mar. 6, 1951 Lubbock Apr. 10, 1951 Navias Aug. 14, 1951 Niehus Nov. 11, 1952 Abbott Sept. 15, 1953 Zaba Oct. 6, 1953 FOREIGN PATENTS France Apr. 9, 1908 France Feb. 1, 1954 Great Britain Oct. 8, 1952 

